1. Field of the Invention
The present invention relates to an image pickup apparatus, an image processing method and a computer program. More particularly, the present invention relates to an image pickup apparatus such as a digital camera which performs a process of eliminating dark current components of an image pickup device, an image processing method and a computer program.
2. Description of Related Art
An image pickup apparatus such as a digital camera has an image pickup device such as complementary metal oxide semiconductor (CMOS) and charge coupled device (CCD), and is configured to generate a photographed image by a process of converting optical signals into electrical signals at the image pickup device.
Image pickup devices equipped in many general cameras are small image pickup devices smaller than a 35 mm film (36×24 mm) for example. However, in recent years, cameras have been developed which are equipped with a large image pickup device having an increased number of effective pixels and enable to photograph high quality images. For example, there is a camera equipped with a large image pickup device such as CMOS and CCD having the same size as a 35 mm film (36×24 mm) for example.
An image pickup device such as CMOS and CCD is made of semiconductor, and is required to form a dense integrated circuit on a silicon substrate by semiconductor manufacture processes including an exposure process. Specifically, a mask pattern of a semiconductor integrated circuit is reduced in scale by utilizing a stepper, and printed on a silicon substrate. However, there is a limit in the size of a stepper, and when a large image pickup device such CMOS and CCD having the same size as a 35 mm film (36×24 mm) is to be manufactured, the image pickup device are divided into a plurality of areas to perform an exposure process in the unit of each divided area, and an integrated circuit of the whole image pickup device is completed by a plurality of exposure processes. Thus, the manufacture of a large image pickup device requires an exposure process for each divisional area.
However, an image pickup device manufactured by a divisional exposure process (step-and-repeat exposure process) causes a problem that the characteristics of each divisional area subjected to each exposure process become different. For example, if an image pickup device is manufactured by dividing it into right and left halves, there is a difference between a dark current of a left image pickup device and a dark current of a right image pickup device, causing a problem that right and left images are become different distinctly.
Generally, in a small image pickup device manufactured by a single exposure process, a dark current in an optical black (OPB) area formed in a peripheral area of the image pickup device is measured, and a dark current is eliminated by a dark current eliminating process on the basis of the measured value.
An image pickup device has a structure having an effective image area for acquiring image data and an optical black (OPB) area formed in a peripheral area of the effective image area and not receiving light. A dark current in the OPB area is measured, and a dark current in the effective image area for receiving light corresponding a photographed image is estimated to eliminate the dark current. This configuration is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2005-312805(Patent Document 1).
An amount of generation of dark current changes also with a temperature, and a dark current eliminating process considering the temperature characteristics is also being studied. For example, Japanese Unexamined Patent Application Publication No. 2005-130045(Patent Document 2) discloses a configuration that a dark current value or dark current noises are measured by using an OPB area and compared with dark current value data obtained beforehand by considering temperature characteristics to estimate a temperature of the image pickup device, and an image correction process (such as gain setting) is executed in accordance with the estimated temperature.
However, an image correction process by an earlier developed dark current elimination is fundamentally a process compatible with a small image pickup device manufactured by a single manufacture process, and does not realize a correction process compatible with a large image pickup device having device areas with different characteristics caused by a plurality of manufacture processes described above.